Inverters are used to convert a DC input voltage into poly-phase AC output voltage for application, for example, to poly-phase stator windings of an electric motor. Advances in power bridge electronics, such as fast switching power semiconductor devices, and advances in control electronics, such as digital signal processors, has led to widespread use of high frequency pulse width modulation techniques of inverter control which allows for relatively precise control of stator voltages and correspondingly precise control over stator currents. Additionally, direct and indirect vector control topologies, which rely upon real-time processing capabilities, have advanced particularly well in light of these advances.
For AC machine drive applications, full utilization of the DC supply voltage is critical in attaining maximum machine output torque and high efficiency. This is particularly important in such applications as vehicle drivelines in electric or electric-hybrid vehicles. It is therefore advantageous that inverter controls make use of output voltage capability of inverters into the non-linear or overmodulation regions. Another competing consideration of substantial importance in vehicle driveline applications is purity of the power output or, put another way, the absence of power output harmonics which are generally undesirable.
Heretofore, known techniques for providing AC output voltages through DC power links and inverter controls have had limited utilization of the DC supply voltage due to unacceptably high harmonic content at the output. This is particularly true in the non-linear or overmodulation region of inverter control. Hence, it is desirable to maximize the DC supply voltage utilization while simultaneously providing a relatively harmonically clean AC output voltage.